Gantry cranes and straddle carriers used in ports are powered by diesel engines or they may have a connection the electric mains. Generally the gantry cranes and straddle carriers are used to pick up cargo containers at the port area for storing them in stacks or for loading the containers for transportation either by sea or land. These operations require lifting the cargo containers and to moving them to a desired location.
The weight of empty cargo containers may be over 2000 kg and their carrying capacity may be up to 40000 kg or more. Lifting such high loads poses a high requirement on the lifting power. When the containers are lifted up to a certain height and then lowered, potential energy is freed. Thereby, due to the weight of the cargo containers both the energy for lifting and lowering is high.
A conventional gantry crane may have a height of up to 7 cargo containers and a width of multiple cargo containers, thus covering multiple rows of cargo containers at a time. The large dimensions provide fast moving of containers between adjacent stacks and in the rows covered by the gantry.
A conventional straddle carrier may have a height of up to 4 cargo containers and a width of approximately 5 metres, thus being wide enough to cover one container at a time. The straddle carrier provides fast and flexible moving of containers between different areas of container terminals.
The power from the engine of the gantry crane and the straddle crane must be fed to the motors which may be located on the top of it, or at its foot, near the tires. Due to the large dimensions and the plurality of functions needing power in the gantry crane and straddle carrier, energy losses between the engine and the different functions may become large.
Hydraulic systems are generally used in cargo handling devices to transfer energy from the engine to the cargo handling. However, spillage of the hydraulic fluids may be environmentally harmful and hydraulic systems are vulnerable to leaks, which cause power leakage and thereby decrease in lifting capacity. Accordingly, also the maintenance costs of hydraulic lifting systems may be high due to short service interval needed to control the leakages.
When heavy cargo containers are lifted to high altitudes a lot of energy is needed. Once lifted to a high altitude, a cargo container has potential energy that is directly proportional to the altitude of the cargo container from the ground. When the cargo container is then lowered to a lower altitude, on top of another cargo container or to the ground, potential energy is released. Due to the large mass of the cargo containers, also the kinetic energy involved in moving them is large. In braking, the kinetic energy is released.
Due to the large mass of conventional gantry cranes and straddle carriers also the kinetic energy involved in their movement is large even without any payload. Thereby, the amount of energy needed in acceleration is large, whereby in deceleration kinetic energy is lost.
Accordingly, due to the high amount of energy involved in operating the gantry crane or straddle carrier, a peak power requirement may be very high while a lot of energy is released in deceleration and/or lowering cargo.
However, loosing high amounts of energy in cargo handling is not environmentally sustainable. With increasing cost of energy, energy efficiency of cargo handling is also an economical issue to companies operating cargo with gantry cranes and other cargo handling vehicles.